yesterday I managed to get an EPUB book working on Adobe Digital Editions AND on my Sony PRS 600.
I continued fiddling and 'improving' the epub and then I broke it. No backup of the working edition of course.

What happens is this:
The book consists of some 15 HTML files and a CSS. I give each of the HTMLs a letter to name them each:

ABCDeFghijklmn ..etc

The uppercase chapters (HTML-files) show up in ADE nicely. When, however I cliick the lowercase chapters, ADE crashes (Taskmanager, abort process etc).
It also happens when I scroll , eg. from D into e or from F into g.

Content.opf and toc.ncx seem to be correct. All images and fonts are there and the TOC (not nested) shows up in ADE (v 1.7.1.1085) correctly.
epubcheck shows no errors and warnings, CSE HTML validator (XHTML 1.1) says all is well and HTMLtidy too.
Book works OK in Firefox epub plu-in.

pffff finally, I found the error. Took me 2 days.
Not reported by any of the validation tools I mentioned, nor by epubcheck. Which shows that we badly need good validation tools. ONE validation tool woold be nice.
Here is what was wrong in the CSS linked into all of the HTMLs. The chapters that worked did NOT have an <h2>.
See the error ?